The invention relates to a hydraulic damper, in particular a motor vehicle hydraulic suspension damper, comprising a tube filled with working liquid, a main piston assembly disposed slidably inside the main section of the tube, attached to a piston rod led outside the damper and dividing the tube into a rebound chamber and a compression chamber, rebound and compression valve assemblies to control the flow of working liquid within the tube during a rebound and compression stroke of the damper, wherein at least one end of the tube is provided with a narrower section of a smaller diameter provided at least partially with at least one axial groove and the damper is further provided with at least one additional piston assembly having diameter smaller than diameter of the main section of the tube, displaceable along with the main piston assembly and apt to be introduced in said conical section of the tube to generate additional damping force.
The additional piston assembly that may slidably displace while inside the narrower section of the tube forms a so called hydraulic stop arrangement that generates additional damping force over a predefined end section of an operating range of piston rod travel.
Exemplary dampers provided with such a hydraulic stop arrangements are disclosed in patent publications U.S. Pat. No. 3,447,644 and EP 2 302 252 A1.
Document U.S. Pat. No. 3,447,644 discloses a damper including subsidiary control means comprising a part positioned adjacent to at least one end of the tube and constituting a subsidiary cylinder and a subsidiary piston on a piston rod arranged to enter said subsidiary cylinder when a main piston assembly approaches such end of the tube, said subsidiary piston and cylinder being a close but free sliding fit and formed to provide a leakage path the cross sectional area of which after entry of the subsidiary piston into engagement with the subsidiary cylinder progressively diminishes with further movement of said subsidiary piston into said subsidiary cylinder. The absorber furthermore comprises subsidiary passage means separate and apart from the rebound and compression valves of the main piston assembly for flow of fluid into and out of said subsidiary cylinder when engaged by said subsidiary piston, and non-return valve means controlling said subsidiary passage means.
European patent application EP 2 302 252 A1 discloses a hydraulic damper with a hydraulic rebound stop. The shock absorber comprising a main tube divided, by a piston rod extending through the extension chamber. The shock absorber is further provided with a hydraulic rebound stop, called HRS fixed in the rebound chamber and comprising additional tube restricting the main tube, a piston freely slidably mounted on the rod and having a diameter adjusted to the additional tube and being provided with at least one fluid-passage substantially axially oriented. The axial displacements of the HRS-piston are limited between a Rebound-stop and a HRS-ring, both fixed to the rod. The fluid-passage is open to a flow of fluid when in abutment against the HRS-ring and being sealed when in abutment against the Rebound-stop. The HRS is further provided with at least one fluid-passage connecting the HRS-chamber to the extension chamber and providing to the fluid a way out for an exiting flow generating a HRS-damping which is tunable and varies as the HRS-piston penetrates the HRS-tube, their relative position determining the size of the way-out.
These known solutions enable for generation of additional damping force depending mainly on the position of a piston rod. In a majority of these solutions a narrower section of the tube has a form of an additional insert of a decreased inner diameter fixed inside the main tube of a damper. This additional component obviously increases the cost of a damper and leads to complications in its assembling process.
Furthermore the increase of a damping force in hydraulic stop arrangements known in the state of art is usually excessively abrupt, which causes various problems (such as additional stresses acting on the HRS-piston sealing rings) during the phase when the additional piston assembly enters the narrower section of the tube.
It has been the object of the present invention to provide a hydraulic damper with a hydraulic stop arrangement of a simple construction providing a smooth increase of damping force which would be cost efficient and simple to manufacture.